1. Field of the Invention
The invention relates to a resonator filter, and more particularly, to a resonator filter in a stacked crystal filter (hereinafter abbreviated as SCF) arrangement.
2. Description of the Prior Art
In recent years, piezoelectric thin film process is prevalently employed for forming filters and duplexer used in radio frequency (RF) communication systems. The conventional piezoelectric thin film acoustic component can be classified as a thin film bulk acoustic resonator (FBAR) and a solidly mounted resonator (SMR).
A resonator filter in SCF arrangement conventionally includes a top electrode, a middle electrode, a bottom electrode, an upper piezoelectric layer sandwiched in between the top and middle electrodes, and a lower piezoelectric layer sandwiched in between the middle and bottom electrodes. For example, when the top electrode serves as the input electrode, the middle electrode serves as the ground electrode, and the bottom electrode serves as the output electrode, the input electrode receives a signal from an input terminal, and the upper piezoelectric layer then generates a bulk acoustic wave to the lower piezoelectric layer in response to the signal excitation. A resonance is therefore generated between the input electrode and the output electrode, and the output electrode outputs the signal to an output terminal.
For such as a RF filter, insertion loss and bandwidth are the key factors to judge filter performance. Therefore, it is always in need to reduce insertion loss and increase bandwidth.